1. Field
This invention pertains to the generation of test signals for receiving systems and, in particular, to the generation of test signals for wideband systems employing harmonic mixing.
2. Prior Art
Frequency scanning superheterodyne receivers are well known. With the development of modern microwave devices, the frequency range of such receivers has been extended to frequencies well beyond 20 GHz. The ability of these receivers to continuously sweep over wide bandwidths has also been extended, as illustrated by the multioctave tuning range of the receiving system described in U.S. Pat. No. 3,701,949.
In some cases, harmonic mixing is used to achieve a wide receiver bandwidth. In this type of mixing, the harmonics of a relatively narrow bandwidth local oscillator are applied to the receiver mixer to cover a wider RF tuning range. For example, a local oscillator with a range of 1.8 to 4.0 GHz may be swept over its range repetitively, while the receiver preselector sweeps continuously over a much wider frequency range of 2 to 12 GHz. For the receiver tuning range of 2 to 4 GHz, the local oscillator sweeps from 1.8 to 3.8 GHz to produce an IF frequency of 200 megacycles. For the receiver RF tuning range of 4 to 8 GHz, the oscillator sweeps over the range of 1.9 to 3.9 GHz, to produce a second harmonic signal which sweeps over the range of 3.8 to 7.8 GHz and again produces an IF frequency of 200 MHz. For the 8 to 12 GHz RF tuning range of the receiver, the third harmonic of the local oscillator produces a 200 MHz IF signal in a similar manner.
Tracking generators suitable for operating with such wideband receivers are generally of two types. In the first of these types, the generator accepts a portion of the sweep control voltage from the receiver, such as the signal applied to drive the preselector. This voltage is applied within the tracking generator to drive an oscillator which covers the full range of the receiver. Unfortunately, oscillators with the extremely wide bandwidths are often not commercially available. An additional problem is the sweep control voltage for the preselector may not be a continuous. A receiver using a harmonically generated local oscillator signal may have a preselector control voltage which stops and retraces a portion of the sweep cycle while the local oscillator is retraced through its complete range. In this type of system, the sweep control voltage may include a retrace portion which can result in drift that prevents the tracking oscillator from properly tracking the receiver.
In the second type of tracking oscillator system, a portion of the local oscillator signal from the receiver is supplied to the tracking generator. An offset oscillator within the generator produces a signal at the frequency to which the receiver is tuned. This type of tracking generator cannot function properly where the receiver local oscillator does not directly produce the local oscillator signal. For example, in a receiver employing harmonic mixing, the harmonics may be produced in the mixer itself rather than in the local oscillator. In this case, it would be difficult to couple a portion of the harmonic signal from the mixer for use in the tracking generator. In addition, the local oscillator harmonic signal produced in the mixer is usually weak and if it is to be applied to the tracking generator it must first be extracted from a large number of signals present in the mixer. The technical difficulties in accomplishing this generally makes the offset oscillator system unsuitable as a tracking generator for wideband receivers employing harmonic mixing.